1. Field of Invention
The present invention relates to resonant power converters, and more particularly to the control circuit of the resonant power converter.
2. Description of Related Art
Power converters are used to convert an unregulated power source to a constant power source. A power converter normally includes a transformer having primary and secondary winding to provide the isolation. Switching devices connected in the primary winding control energy transferring from the primary winding to the secondary winding. The power converters operate at high frequencies allows a size and weight reduction. However, the switching losses, component stresses, and electric-magnetic-interference (EMI) are inherent problems. In the last decades, many resonant power converters have been proposed for high frequency power conversion to reduce the switching losses. Among them, the resonant switching techniques are described in following prior arts: “Regulated switched power circuit with resonant load” by Tamas S. Szepesi, U.S. Pat. No. 4,535,399; “Frequency controlled resonant regulator” by Frank S. Wendt, U.S. Pat. No. 4,631,652.
For the power saving at no load, a burst switching schemes is developed for resonant power control, “Full load to no-load control for a voltage fed resonant inverter” by John N. Park and Robert L. Steigerwald, U.S. Pat. No. 4,541,041. In various resonant power converters, the leakage inductance of the transformer or additional magnetic components is employed as a resonant inductor to generate the circulating current for achieving the soft switching.
However, there still exist several drawbacks in conventional technologies. The resonant inductor associated with a resonant capacitor forms a resonant circuit for the power converter to deliver the maximum power to the load. Although the switching frequency of the power converter is controlled to higher or lower than the resonant frequency in response to the change of the load, the change of the switching frequency must be restricted to ensure the linear operation and achieve the soft switching of the resonant power converter. Furthermore, the poor audio-susceptibility of the power converter and the acoustic noise of the burst switching are disadvantages. These shortcomings are the main object of the present invention to overcome.